Class of stabilizing compounds for phosphor screens

ABSTRACT

A phosphor screen is provided comprising a satiblizer being an organic compound or an oligomeric or polymeric compound comprising moieties corresponding to the following general formula I: ##STR1## wherein: R 1 , R 2 , R 3  and R 4  each independently represent a substituted or unsubstituted alkyl- or aryl group, 
     R 5  represents hydrogen or a substituted or unsibstituted alkyl group and 
     Z represents the atoms necessary to form a substituted or unsubstituted 5 to 8 membered ring.

DESCRIPTION

1. Field of the Invention

The invention relates to X-ray intensifying screens and stimulablephosphor screens. More particularly it relates to compositions thatincrease the stability of both X-ray intensifying screens and stimulablephosphor screens.

2. Background of the Invention

In a conventional radiographic system an X-ray radiograph is obtained byX-rays transmitted imagewise through an object and converted into lightof corresponding intensity in a so-called intensifying screen (X-rayconversion screen) wherein phosphor particles absorb the transmittedX-rays and convert them into visible light and/or ultraviolet radiationto which a photographic film is more sensitive than to the direct impactof X-rays.

In practice the light emitted imagewise by said screen irradiates acontacting photographic silver halide emulsion layer film which afterexposure is developed to form therein a silver image in conformity withthe X-ray image.

For use in common medical radiography the X-ray film comprises atransparent film support, coated on both sides with a silver halideemulsion layer. During the X-ray irradiation said film is arranged in acassette between two X-ray conversion screens each of them makingcontact with its corresponding silver halide emulsion layer.

According to another method of recording and reproducing an X-raypattern disclosed e.g. in U.S. Pat. No. 3,859,527 a special type ofphosphor is used, known as a photostimulable phosphor, which beingincorporated in a screen is exposed to incident pattern-wise modulatedX-rays and as a result thereof temporarily stores therein energycontained in the X-ray radiation pattern. At some interval after theexposure, a beam of visible or infra-red light scans the screen tostimulate the release of stored energy as light that is detected andconverted to sequential electrical signals which are processable toproduce a visible image. For this purpose, the phosphor should store asmuch as possible of the incident X-ray energy and emit as little aspossible of the stored energy until stimulated by the scanning beam.

In both systems phosphor particles are dispersed in a binder and bycoating the dispersion of phosphor particles on a support a supportedscreen is formed, or by casting the dispersion a self-supporting screenis formed. The screens have to withstand not only physical strains butalso strain due to environmental causes especially humidity and heat.Due to humidity and heat, some phosphors, contained in the screen tendto decompose. This decomposition can be homogeneous, e.g. a colorationover all the surface of the screen, thus diminishing the speed of theX-ray system by absorbing emitted light (promptly emitted light as wellas stimulated emission light). The deterioration of the screen due tophosphor decomposition can be local, thus diminishing the image qualityof the system because in some place no image or a blurred image isformed.

It has been described in GB 1,575,511 that the derioration of X-rayintensifying screens comprising a halide-containing rare earth metalphosphor in which the host metal is a rare earth metal and the activatoris one or more other rare earth metals or a barium fluoride chloridephosphor activated by europium(II) or a sodium activated CsI phosphorcould be diminished by the addition to the screen of metal-organicsubstances. Although this disclosure claims the use of metal-organicsubstances, the examples of this disclosure are mainly focussed onorganotin substances.

In EP-A 234 385 it is disclosed that a storage screen (i.e. a stimulablephosphor screen) comprising a iodine containing bariumflorohalide fosforcould be stabilized by the addition of organotin compounds and orcompounds having an epoxygroup.

Up until now the stabilizing compounds had in practice to be organotincompounds. This restriction limits the degrees of freedom when preparingintensifying screens or stimulable phosphor screens. The binder, solventand further ingredients of the coating solution of such a screen hadalways to be chosen so that there always was a compatibility withorganotin compounds. Therefore it is still desirable to find furtherstabilizing compounds for use in both X-ray intensifying screen and(photo)stimulable screens.

3. OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to provide screens comprising promptemitting phosphor particles, as well as screens comprising stimulablephosphor particles, that are basically insensitive to environmentalinfluences.

It is another object of the invention to provide screens, as describedabove, that are stabilized against the influence of moisture.

It is an object of the invention to provide further means forstabilizing screens comprising prompt emitting phosphor particles orstimulable phosphor particles so that the degree of freedom duringmanufacture of said screens is enhanced.

Further objects and advantages will become clear from the detaileddescription hereinafter.

The objects of the invention are realized by providing a phosphorscreen, characterised in that said screen comprises at least onestabilizer being an organic compound or an oligomeric or polymericcompound comprising moieties corresponding to the following generalformula I: ##STR2## wherein:

R¹, R², R³ and R⁴ each independently represent a substituted orunsubstituted alkyl- or aryl group,

R⁵ represents hydrogen or a substituted or unsubstituted alkyl group and

Z represents the atoms necessary to form a substituted or unsubstituted5 to 8 membered ring

In a preferred embodiment said screen comprises at least one organiccompound or a oligomeric or polymeric compound comprising moietiescorresponding to the following general formula II: ##STR3## wherein:

R¹, R², R³ and R⁴ each independently represent a substituted orunsubstituted alkyl- or aryl group,

R⁵ represents hydrogen or a substituted or unsibstituted alkyl group andX an arbitrary substitution.

4. DETAILED DESCRIPTION OF THE INVENTION

It was surprisingly found that screens, comprising prompt emittingphosphor particles as well as stimulable phosphor particles,(hereinafter the wording phosphor screen is used to include bothscreens, comprising prompt emitting phosphor particles and screenscomprising stimulable phosphor particles) could be stabilized againstdegradation due to environmental influences (temperature, humidity,etc.) by the addition to the phosphor comprising layer of said screensof compounds having a ring structure comprising a --C--NH--C--sequencewherein the two carbon atoms in the vicinity of the NH group each aresubstituted by two alkyl or aryl groups. Said ring compounds may besingle compounds as well as oligomeric or polymeric compounds comprisingsaid ring structure as moieties making up (optionally with othermoieties) said oligomeric or polymeric compounds. The ring structurecorresponds to general formula I: ##STR4## wherein:

R¹, R², R³ and R⁴ each independently represent a substituted orunsubstituted alkyl- or aryl group,

R⁵ represents hydrogen or a substituted or unsibstituted alkyl group and

Z represents the atoms necessary to form a substituted or unsubstituted5 to 8 membered ring.

It is preferred to use, in a screen according to the present invention,piperidine derivatives that carry two alkyl or aryl groups in the 2position as well as in the 6 position. Said piperidine derivatives canbe single compounds as well as oligomeric or polymeric compoundscomprising piperidine moieties that carry two alkyl or aryl groups inthe 2 position as well as in the 6 position. In a general formula saidpiperidine compounds correspond to formula II: ##STR5## wherein:

R¹, R², R³ and R⁴ each independently represent a substituted orunsubstituted alkyl- or aryl group.

R⁵ represents hydrogen or a substituted or unsibstituted alkyl group andX an arbitrary substitution. Most preferred piperidine derivativescorrespond to formula III, wherein the arbitrary substitution X is inthe 4-position: ##STR6## Specific examples of very useful compounds,according to the present invention, having a ring structure comprising a--C--NH--C-- sequence wherein the two carbon atoms in the vicinity ofthe NH group each are substituted by two alkyl or aryl groups arecompounds A1 to A3. A1 is an example of a single compound, A2 is acompound comprising 2 moieties having a ring structure comprising a--C--NH--C-- sequence wherein the two carbon atoms in the vicinity ofthe NH group each are substituted by two alkyl or aryl groups and A3 isa polymeric compound comprsing moieties A3a and A3b. ##STR7##

The compounds having a ring structure comprising a --C--NH--C-- sequencewherein the two carbon atoms in the vicinity of the NH group each aresubstituted by two alkyl or aryl groups are mixed before the manufactureof the screen with the dispersion of the phosphor particles in a solventand optionally a binder. The stabilizing compounds according to thepresent invention are adden in an amount of 10⁻² mole % to 5 mole % withrespect to the phosphor. Preferably said compounds are mixed in anamount between 0.2 mole % and 2 mole % with respect to the phosphor.

The compounds, according to this invention, having a ring structurecomprising a --C--NH--C-- sequence wherein the two carbon atoms in thevicinity of the NH group each are substituted by two alkyl or arylgroups can be used to stabilize any phospor, both stimulable (storagephosphors) and prompt emitting phosphors. Typical examples of storagephosphors employable in a radiation image storage screen of the presentinvention include:

SrS:Ce, Sm, SrS:Eu, Sm, ThO₂ :Er, and La₂ O₂ S:Eu, Sm, as described inU.S. Pat. No. 3,859,527;

ZnS:Cu,Pb, BaO.xAl₂ O₃ :Eu, in which x is a number satisfying thecondition of 0.8<x<10, and M²⁺ O.xSiO₂ :A, in which M²⁺ is at least onedivalent metal selected from the group consisting of Mg, Ca, Sr, Zn, Cdand Ba, A is at least one element selected from the group consisting ofCe, Tb, Eu, Tm, Pb, Tl, Bi and Mn, and x is a number satisfying thecondition of 0.5<x<2.5, as described in U.S. Pat. No. 4,326,078;

M^(III) OX:xCe, in which M^(III) is at least one trivalent metalselected from the group consisting of Pt, Nd, Pm, Sm, Eu, Tb, Dy, Ho,Er, Tm, Yb and Bi; X is at least one element selected from the groupconsisting of Cl and Br; and x is a number satisfying the condition of0<x<0.1, as described in Japanese Patent Provisional Publication No.58(1983)-69281;

LnOX:xA, in which Ln is at least one element selected from the groupconsisting of La, Y, Gd and Lu, X is at least one element selected fromthe group consisting of Cl and Br, A is at least one element selectedfrom the group consisting of Ce and Tb, and x is a number satisfying thecondition of 0<x<0.1, as described in the above-mentioned U.S. Pat. No.4,236,078;

(Ba_(1-x),M^(II) _(x))FX:yA in which M^(II) is at least one divalentmetal selected from the group consisting of Mg, Ca, Sr, Zn and Cd, X isat least one element selected from the group consisting of Cl, Br and I,A is at least one element selected from the group consisting of Eu, Tb,Ce, Tm, Dy, Pt. Ho, Nd, Yb and Er, and x and y are numbers satisfyingthe conditions of 0<x<0.6 and 0<y<0.2 respectively, as described in U.S.Pat. No. 4,239,968.

Bariumfluorohalide phosphors as disclosed in, e.g., U.S. Pat. No.4,239,968, DE OS 2 928 245, U.S. Pat. No. 4,261,854, U.S. Pat. No.4,539,138, U.S. Pat. No. 4,512,911, EP 0 029 963, U.S. Pat. No.4,336,154, U.S. Pat. No. 5,077,144, U.S. Pat. No. 4,948,696, JapanesePatent Provisional Publication No. 55(1980)-12143, Japanese PatentProvisional Publication No. 56(1981)-116777, Japanese Patent ProvisionalPublication No. 57(1982)-23675, U.S. Pat. No. 5,089,170 U.S. Pat. No.4,532,071, DE OS 3 304 216, EP 0 142 734, EP 0 144 772, U.S. Pat. No.4,587,036, U.S. Pat. No. 4,608,190, and EP 0 295 522.

Ba_(1-x) Sr_(x) F_(2-a-b) Br_(a) X_(b) :zA, wherein X is at least onemember selected from the group consisting of Cl and I; x is in the range0.10<x<0.55; a is in the range 0.70<a<0.96; b is in the range 0<b<0.15:z is in the range 10⁻⁷ <z<0.15, and A is Eu²⁺ or Eu²⁺ together with oneor more of the co-dopants selected from the group consisting of Eu³⁺, Y,Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb, Er, La, Gd and Lu, and wherein fluorineis present stoichiometrically in said phosphor in a larger atom % thanbromine taken alone or bromine combined with chlorine and/or iodine, asdisclosed in EP 345 903.

Alkali metal phosphors comprising earth alkali metals as disclosed ine.g. U.S. Pat. No. 5,028,509 and EP 0 252 991. Alkalimetal phosphors ase.g. RbBr:Tl.

Halosilicate phosphors as disclosed in, e.g.,EP 304 121, EP 382 295 andEP 522 619.

Elpasolite phosphors as disclosed in European Application 94201578 filedon Jun. 17, 1994.

The compounds, according to the present invention, having a ringstructure comprising a --C--NH--C-- sequence wherein the two carbonatoms in the vicinity of the NH group each are substituted by two alkylor aryl groups, can especially well be used to stabilize storagephosphor screens comprising a storage phosphor containing iodine, andare especially well suited for stabilizing bariumfluorohalide phosphorscomprising iodine.

The compounds, according to the present invention, can be used tostabilize screens comprising any prompt emitting phosphor, e.g. CaWO₄,BaFCl, BaFBr, Ba(F,CI,I)₂, Ba(F,Br,I)₂, Y-tantalate phosphors asdescribed in e.g. EP-A 202 875, Gd₂ O₂ S, LaOX phosphors, where X is oneof Cl, Br or I, etc. with or without dopants. Thezinc-dithiocarboxylates can especially well be used to stabilize promptemitting X-ray screens comprising a prompt emitting phosphor being amember selected from the group consisting of a halide-containing rareearth metal phosphor in which the host metal is a rare earth metal andthe activator is one or more other rare earth metals, a barium fluoridechloride or bromide phosphor activated by europium(II), a bariumfluoride chloride or bromide phosphor activated by europium(II)comprising iodine and a sodium activated CsI phosphor.

The screens comprising stabilizing compounds according to the presentinvention can be self supporting as well as supported. A supported and aself supporting screen comprise preferably a binder layer incorporatingphosphor particles in dispersed form. The binder is preferably at leastone film forming organic polymer, e.g. a cellulose acetate butyrate,polyalkyl (meth)acrylates, e.g. polymethyl methacrylate, apolyvinyl-n-butyral e.g. as described in the U.S. Pat. No. 3,043,710, acopoly(vinyl acetate/vinyl chloride) and acopoly(acrylonitrile/butadiene/styrene) or a copoly(vinyl chloride/vinylacetate/vinyl alcohol) or mixture thereof.

When a binder is used, it is most preferred to use a minimum amount ofbinder. The weight ratio of phosphor to binder preferably from 80:20 to99:1. The ratio by volume of phoshor to binding medium is preferablymore than 85/15.

Preferably the binding medium substantially consists of one or morehydrogenated styrene-diene block copolymers, having a saturated rubberblock, as rubbery and/or elastomeric polymers as disclosed in WO94/00531. Particularly suitable thermoplastic rubbers, used asblock-copolymeric binders in phosphor screens in accordance with thisinvention are the KRATON-G rubbers, KRATON being a trade mark name fromSHELL.

The coverage of the phosphor is preferably in the range from about 5 and250 mg/cm², most preferably between 20 and 175 mg/cm².

When the phosphor layer, comprising a stabilizing compound according tothe present invention, is used as a supported layer on a support sheet,said support is made of a film forming organic resin, e.g. polyethyleneterephthalate, but paper supports and cardboard supports optionallycoated with a resin layer such as an alpha-olefinic resin layer are alsoparticularly useful. Further are mentioned glass supports and metalsupports. The thickness of the phosphor layer is preferably in the rangeof 0.05 mm to 0.5 mm.

Screens comprising stabilizing compound according to the presentinvention, can be prepared by intimately mixing the phosphor in asolution of the binder and then coating on the support and drying. Thecoating of the present phosphor binder layer may proceed according toany usual technique, e.g. by spraying, dip-coating or doctor bladecoating. After coating, the solvent(s) of the coating mixture is (are)removed by evaporation, e.g. by drying in a hot (60° C.) air current.

An ultrasonic treatment can be applied to improve the packing densityand to perform the de-aeration of the phosphor-binder combination.Before the optional application of a protective coating thephosphor-binder layer may be calandered to improve the packing density(i.e. the number of grams of phosphor per cm³ of dry coating).

Optionally, a light-reflecting layer is provided between thephosphor-containing layer and its support to enhance the output of lightemitted by photostimulation. Such a light-reflecting layer may containwhite pigment particles dispersed in a binder, e.g. titanium dioxideparticles, or it may be made of a vapour-deposited metal layer, e.g. analuminium layer, or it may be a coloured pigment layer absorbingstimulating radiation but reflecting the emitted light as described e.g.in U.S. Pat. No. 4,380,702.

In order to improve resolution it is possible to provide underneath thephosphor layer a layer absorbing the emitted light e.g. a layercontaining carbon black or to use a coloured support e.g. a grey orblack film support.

EXAMPLES STABILIZING STIMULABLE PHOSPHOR SCREENS 1. PREPARATION OF THESTIMULABLE PHOSPHORS

All stimulable phosphor samples have been prepared in the following way:

The phosphor precursors:

BaF₂ : 0.819 mol

SrF₂ : 0.18 mol

NH₄ Br: 0.82 mol

NH₄ I: 0.15 mol

EuF₃ : 0.001 mol

CsI: 0.003 mol.

forming a raw mix, were collected in a PE container, and the mix washomogenized for 15' on a jarr rolling mill. Next, the powder mix wastransferred to a rotating blade mixer (Henschel Germany) and milled for5' at 2,000 rpm under Ar atmosphere. Three crucibles containing 130 g ofthe mix each, were placed in a quartz tube. The quartz tube was sealedwith a flange with a water lock at the gas outlet side. The sealedquartz tube was placed in an oven at 850° C., and the temperature waskept constant at this temperature during the three hour firing. Duringthe firing the tube was flushed with Ar at a rate of 1.5 l/min. Afterthe firing, the tube was taken out of the furnace and allowed to cool.After the cooling, the flange was removed and the three crucibles weretaken out of the tube. The powder was milled and homogenized. A secondfiring was performed at 750° C., for 6 hours, under a 1.5 l/min 99.8% N₂/0.2% H₂ gas flow rate. Finally, the powder was deagglomerated with apestle and mortar. The phosphor corresponded to the formula:

    Ba.sub.0.8175 Sr.sub.0.18 Eu.sub.0.001 Cs.sub.0.0015 F.sub.1.19 Br.sub.0.70 I.sub.0.11

2. PREPARATION OF THE STIMULABLE PHOSPHOR SCREENS

The phosphor was dispersed in a mixture of celluloseacetobutyrate/polyethylacetate binder. The solvents of the dispersionwere methylethylketine, ethyleneglycolomonoethylether and ethylacetate.The phosphor was present in this dispersion a rato of 35% by weight withrespect to the total weight of the dispersion. The dispersion is coatedwith a bar coater on a support, such as to have 1000 g phosphor per m².In the finished screen, the phosphor was present in 89% by weight withrespect to the total weight of phosphor+binder. The screens, except thecomparative screen comprised 1% by weight with respect to the weight ofthe phosphor of the stabilizing compounds.

3. MEASUREMENT OF STABILIZING EFFECT

The screens are subjected to an heat treatment of 45 minutes at 60° C.After this treatment the performance of the various screen was measuredaccording to the procedure described herebelow. The phosphor screen isexcited with an X-ray source operating at 85 kVp and 20 mA. For thatpurpose the Nanophos X-ray source of Siemens AG-W Germany was used. Thelow energy X-rays are filtered out with a 21 mm thick aluminum plate toharden the X-ray spectrum. After X-ray excitation the phosphor screen istransferred in the dark to the measurement setup. In this setup laserlight is used to photostimulate the X-ray irradiated phosphor screen.The laser used in this measurement is a CW Single Mode He-Ne laser withpower 30 mW.

The laser-optics comprise an electronic shutter, a beam-expander and afilter. A photomultiplier (Hamamatsu R 1398) collects the light emittedby the photostimulation and gives a corresponding electrical current.The measurement procedure is controlled by a Hewlett Packard BasicController 382 connected to a HP 6944 multiprogrammer. Afteramplification with a current to voltage convertor a TEKTRONIX TDS420digital oscilloscope visualizes the photocurrent obtained. When theelectronic shutter is opened the laser beam begins to stimulate thephosphor screen and the digital oscilloscope is triggered. Using adiagfraghme placed in contact with the screen the light emitted by only7 mm² is collected. Only half the laser power reaches the screensurface. In this way the intensity of the stimulating beam is moreuniform.

The signal amplitude from the photomultiplier is linear with theintensity of the photostimulating light and with the storedphotostimulable energy. The signal decreases with a power law. When thesignal curve is enterred the oscilloscope is triggered a second time tomeasure the offset which is defined as the component of error that isconstant and independent of inputs. After subtracting this offset thepoint at wich the signal reaches l/e of the maximum value is calculated.The integral below the curve is then calculated from the start to thisl/e point. The function is described mathematyically by f(t)=A.e^(-t/)τ: wherein A is the amplitude, τ is the time constant, t is stimulationtime, and e is the base number of natural logarithms.

The l/e point is reached when t=τ at which 63% of the stored energy hasbeen released. To obtain said result, the computer multiplies theintegral with the sensitivity of the system. The sensitivity of thephotomultiplier and amplifier have therefore to be measured as afunction of anode-cathode voltage of the photomultiplier and theconvolution of the emission spectrum of the phosphor and thetransmission spectrum of the separating filter has to be calculated.Because the emission light is scattered in all directions only afraction of the emitted light is detected by the photomultiplier. Theposition of the screen and photomultiplier are such that 10% of thetotal emission is detected by the photomultiplier. After all thesecorrections have been made a conversion efficiency value (C.E.) isobtained in pJ/mm³ /mR. This value varies with screen thickness andtherefore for measurements to be comparable they have to be carried outat constant phosphor coverage.

The stimulation energy (S.E.) is defined as the energy necessary tostimulate 63% of the stored energy and is expressed in μJ/mm².

From the values C.E. and S.E. a figure of merit (F.O.M.) that is a valuedescribing the sensitivity of the phosphor for practical use.F.O.M.=1000×C.E/S.E..

COMPARATIVE EXAMPLE 1 (Cl)

A screen was prepared as described above, without the additon of anystabilizing substance. The performance of the screen was measured interms of CE, SE and FOM.

EXAMPLES 1-3 (E1-E3)

For these examples, screens were prepared as describe above with theaddition of stabilizing substances. The performance of the screens wasmeasured in terms of C.E.2, S.E. and FOM.

The stabilizing substance used and the performance of the variousscreens are listed in table 1.

                  TABLE 1                                                         ______________________________________                                                        C.E.2        S.E                                              Nr.   Stabilizer                                                                              (pJ/mm.sup.3 /mR)                                                                          (μJ/mm.sup.2)                                                                       FOM                                     ______________________________________                                        C1    none      1.5          30       50                                      E1    A1        44           14       3080                                    E2    A2        31           16       1980                                    E3    A3        58           15       4000                                    ______________________________________                                    

STABILIZING PROMPT EMITTING PHOSPHOR SCREENS

Prompt emitting phosphor screen were made by dispersing a LaOBr:Tmphosphor in mixture of solvent and KRATON G (trade mark name fromSHELL). The mixture was coated on a polyethyleneterephthalate filmsupport so that 90 mg phosphor was present per cm².

Two screens were prepared: C2 without any stabilizer, E4 with stabilizerA2.

MEASUREMENT OF STABILITY AGAINST HUMIDITY

The screen were cut into 4 seperate pieces. Four of these pieces wereplaced in an exsiccator filled with water that is maintained at 60° C.The relative humidity is 100%. After 6, 12, 18 and 24 one piece of thescreen was taken out of the exsiccator and dried. After day 18, thethree treated samples of the screen and one untreated sample of thescreen were combined with a Blue sensitive X-ray film (AGFA CURIX RP1,trade name of Agfa-Gevaert NV, Mortsel, Belgium) and the sandwichexposed to X-rays of 70 kVp at a film focus distance of 1.5 m. Thefilm/screen combination is exposed such as to give a density of 1.5(±0.3) D on the film under the untreated screen sample. The differencein density of the film under the untreated sample and the density underthe treated samples is taken as a measure of stability. The smaller thedensity difference, the more stable the screen. The results are given inTable 2.

                  TABLE 2                                                         ______________________________________                                        Nr     Stabilizer      ΔD*                                                                             ΔD**                                     ______________________________________                                        C2     none            -0.285  -0.375                                         E4     A2              -0.240  -0.315                                         ______________________________________                                         *difference in density under the untreated screen and under the screen        treated for 12 days.                                                          **difference in density under the untreated screen and under the screen       treated for 18 days.                                                     

We claim:
 1. A phosphor screen, characterised in that said screencomprises at least one stabilizer being an organic compound or anoligomeric or polymeric compound comprising moieties corresponding tothe following general formula I: ##STR8## wherein: R¹, R², R³ and R⁴each independently represent a substituted unsubstituted alkyl- or arylgroup,R⁵ represents hydrogen or a substituted or unsibstituted alkylgroup and Z represents the atoms necessary to form a substituted orunsubstituted 5 to 8 membered ring.
 2. A phosphor screen according toclaim 1, wherein said stabilizer is an organic compound or a oligomericor polymeric compound comprising moieties corresponding to the followinggeneral formula II: ##STR9## wherein: R¹, R², R³ and R⁴ eachindependently represent a substituted or unsubstituted alkyl- or arylgroup,R⁵ represents hydrogen or a substituted or unsibstituted alkylgroup and X an arbitrary substitution.
 3. A phosphor screen according toclaim 1, wherein said stabilizer is present in an amount of 10⁻² mole %to 5 mole % with respect to the phosphor.
 4. A phosphor screen accordingto claim 1, wherein said screen comprises a stimulable phosphor.
 5. Aphosphor screen according to claim 4, wherein said stimulable phosphoris a bariumfluorohalide phosphor comprising iodine.
 6. A phosphor screenaccording to claim 1, wherein said screen comprises a prompt emittingphosphor.
 7. A phosphor screen according to claim 6, wherein said promptemitting phosphor is a member selected from the group consisting of ahalide-containing rare earth metal phosphor in which the host metal is arare earth metal and the activator is one or more other rare earthmetals, a barium fluoride chloride or bromide phosphor activated byeuropium(II), a barium fluoride chloride or bromide phosphor activatedby europium(II) comprising iodine and a sodium activated CsI phosphor.